1. Field of the Invention
The present invention relates to an electronic device and a method for manufacturing the same. In particular, the present invention relates to a solar cell and a method for manufacturing the same, for example.
2. Description of Related Art
Oxide thin films containing an element from group IIa and an element from group IIb are used effectively for devices such as light-emitting elements and solar cells because they can be treated as a semiconductor and their band gap can be controlled by the content of the element from group IIa. In the present specification, each group is named according to an IUPAC short-form periodic table. Incidentally, group IIa and group IIb respectively correspond to group 2 and group 12 in an IUPAC long-form periodic table. In particular, the oxide thin films containing an element from group IIa and an element from group IIb are preferable as a window layer (sometimes referred to as a buffer layer instead) joined to a CIGS film in a thin-film solar cell using CuInSe2 (CIS) or Cu(In, Ga)Se2 (CIGS), which is a solid solution of CIS with Ga, as a light-absorption layer. CIS and CIGS are compound semiconductors (having a chalcopyrite structure) comprising at least one element from each of groups Ib, IIIb and VIb. Group Ib, group IIIb and group VIb respectively correspond to group 11, group 13 and group 16 in the long-form periodic table. An oxide thin film (a semiconductor thin film) containing an oxide of an element from group IIa and an element from group IIb, represented by Zn1−xMgxO disclosed in JP 2000-323733 A, is used effectively for increasing a conversion efficiency of a solar cell because its conduction band can be matched with that of the CIGS film by changing the content of the element from group IIa such as Mg.
Although the band gap can be controlled in the oxide thin film containing elements from groups IIa and IIb, it is difficult to control a valence band level because the group IIa has a predominant influence on the change in a conduction band level. On the other hand, in a material used in an electronic device, for example, a light-emitting element with a multiple well structure, it is important to control not only the conduction band level but also the valence band level. Further, in order to achieve various functions of the electronic device, an electrical conductivity (a carrier density) of the oxide thin film sometimes needs to be controlled.
Also, since the oxide thin film (semiconductor thin film) containing the oxide of an element from group IIa and an element from group IIb required as the window layer of the solar cell is insulative or highly resistive, it is necessary to use high-frequency sputtering as a manufacturing method. In general, when an insulating layer or a high-resistance film is formed by high-frequency sputtering, there is a disadvantage of slow film forming speed. Particularly, the solar cell has to be manufactured in a large area and at a high speed, and the speed of forming the window layer may determine the speed of the solar cell manufacturing process, serving as a factor limiting the production capacity.